High court rules 5-4 that police need a search warrant to obtain information from cellphone towers.

The Supreme Court struck a blow for privacy in the digital age Friday. Justices ruled 5-4 that the cell phone location data used to convict Timothy Carpenter of armed robbery – obtained by prosecutors from his wireless carrier – is subject to the protection of the Fourth Amendment of the Constitution.

The question immediately before the high court in the case of Carpenter v. United States was whether law enforcement needs a search warrant to obtain such data – in this case, detailing Carpenter’s movements over 127 days. The court said a search warrant was needed. It also began to answer the larger, more profound question hanging over the case: will our constitutional right to privacy survive the technological advances of the modern era?

Rapid technological change inevitably outpaces the glacial evolution of the law and the Carpenter case is a perfect example. The location data in question was obtained under the Stored Communications Act (SCA), which did not require prosecutors to meet the "probable cause" standard of a warrant.

The SCA, enacted in 1986 when cell phones barely existed, is unsurprisingly inadequate to protect of our privacy rights in the age of mobile devices.

So Timothy Carpenter turned to the Constitution. But the Justice Department argued that the Fourth Amendment didn't apply because of the Supreme Court's Third-Party Doctrine. That doctrine holds that no search or seizure occurs when the government obtains data that the accused has voluntarily conveyed to a third party – in this case, one's wireless provider.

The Third-Party Doctrine made some sense when it was invented 40 years ago. However, when applied to today's modern technology, the doctrine results in a gaping hole in the Fourth Amendment.

As the Committee for Justice pointed out in an amicus brief to the Court in in the Carpenter case: "Incredibly deep reservoirs of information are constantly collected by third-party service providers today. … This trend will only accelerate as the 'Internet of Things' supplies data revealing more and more of our activities – even use of our household appliances—to third-party service providers."

The good news is that the Supreme Court took a big step towards repairing that hole Friday. In an opinion by Chief Justice John Roberts, the court acknowledged that Fourth Amendment doctrines must evolve to account for “seismic shifts in digital technology.”

Accordingly, the court concluded that the voluntary conveyance assumption behind the Third-Party Doctrine just doesn't hold up when it comes to cell phone location data, because "a cell phone logs a cell-site record by dint of its operation, without any affirmative act on the part of the user beyond powering up."

More generally, the justices recognized that individuals “have a reasonable expectation of privacy in the whole of their physical movements. ... Allowing government access to cell-site records contravenes that expectation. ... When the Government tracks the location of a cell phone it achieves near perfect surveillance, as if it had attached an ankle monitor to the phone’s user."

The Supreme Court also said in the ruling: "Our decision today is a narrow one. We do not express a view on (scenarios) not before us."

Nonetheless, the scenario before the court is a common one. In 2016, AT&T alone received more than 70,000 demands for location data.

The Supreme Court's caution is wise. The justices reminded us of the need to "tread carefully in such cases, to ensure that we do not embarrass the future.”

Nonetheless, the high court's reasoning logically applies to a variety of current and future technologies, such as Internet service and the emerging Internet of Things. Thus, the ramifications of Friday's decision are anything but narrow.

While the Carpenter ruling came in a 5-4 decision – with the four liberal justices joining Chief Justice John Roberts in the majority – it produced a 6-3 majority in favor or protecting cell phone location data.

Justice Neil Gorsuch dissented for technical reasons, but he wrote that he would have gone further than the majority, criticizing the majority for keeping the Third-Party Doctrine "on life support" in the digital age. Based on Gorsuch's record as a lower court judge, he was expected to be a conservative voice for a robust Fourth Amendment. So far, he has not disappointed.

That brings us back to the larger forward-looking question surrounding this case, specifically the prospects for preserving our constitutional right to privacy in the face of rapid technological change. Those prospects brightened Friday as the Supreme Court made it clear that opting out of modern society should not be a requirement for enjoying the protection of the Fourth Amendment.

Curt Levey is president of the Committee for Justice, a nonprofit organization that advocates for the rule of law. Before attending law school, he worked as a scientist in the field of artificial intelligence.